The present invention relates to a device and a method for converting flow of charge into a frequency signal.
Such a device, also known as a charge-frequency converter, is an electronic circuit that is used to measure flow of electric charge.
In a known circuit arrangement of a charge-frequency converter, an integrator and a comparator are connected in series. Electric charge is supplied to the arrangement on the input side via an input of the circuit and is integrated by the integrator. The integrator supplies to the amplifier a voltage signal which corresponds to the integral of the supplied charge. The integrated voltage signal is compared with a pre-set charge comparison value in the comparator. If the signal has reached the comparison value in terms of magnitude, the comparator emits a pulse to the output of the arrangement. The integrator is then reset, and the sequence of integration and comparison begins afresh. At the output of the arrangement, therefore, there is produced over time a sequence of pulses, the instantaneous frequency of which is a measure of the amount of electric charge supplied per unit of timexe2x80x94that is to say flow of charge.
A disadvantage of that arrangement is that the integrator cannot operate during the resetting operation, that is to say there is a dead time during which flow of charge cannot be measured. In time-critical applications, that is to say those in which the amount of charge supplied changes rapidly and has to be detected very quickly, too much time is lost by the resetting of the integrator in the circuit. This can lead to inaccurate measurements, which cannot be tolerated especially in the case of applications in which safety is a consideration, such as, for example, ion beam therapy.
The aim of the invention is therefore to provide a device and a method for converting flow of charge into a frequency signal that ensure exact, quick and reliable measurement of the charge. That aim is achieved by a device having the features according to claims 1 and 14 and by a method having the features according to claims 8 and 21. Advantageous developments are defined in the subsidiary claims.
The device according to the invention and the method according to the invention are associated with a number of advantages.
By virtue of the provision of two parallel circuit branches, when the flow of charge in one circuit branch has reached the pre-set threshold value and the associated integrator has to be reset, the measurement of the flow of charge can be continued in the other circuit branch. The dead time during the measurement for the resetting of the integrator is therefore eliminated.
The provision of various threshold values for each comparator results in a circuit have an extendable measuring range. In addition, the circuit branches are switched less often, so that the switching times and dead times associated with those switching operations are also minimized.
By connecting to the other circuit branch even while measurement is taking place in the one circuit branch it is also possible to eliminate the remaining switching time on the transition from one branch circuit to the other. There are thus provided a device and a method with which flow of charge can be measured continuously, that is to say without dead times.
In addition, the circuit is constructed from commercially available components, and can therefore be produced economically. In addition, the design characteristics of the circuit can be reliably reproduced in large-scale production.